Process for the joint production of sodium carbonate and sodium bicarbonate

ABSTRACT

Process for the joint production of sodium carbonate and sodium bicarbonate crystals, according to which: a solid powder derived from sodium sesquicarbonate, having a mean particle diameter comprised between 0.1 and 10 mm is dissolved in water; the resulting water solution is introduced into a crystallizer, wherein a first water suspension comprising sodium carbonate crystals is produced; the first water suspension is subjected to a separation, in order to produce crystals comprising sodium carbonate on the one hand, which are valorized, and a mother liquor on the other hand; and a part of the mother liquor is taken out of the crystallizer and put into contact in, a gas liquid contactor, with a gas comprising carbon dioxide, in order to produce a second water suspension comprising sodium bicarbonate crystals, which are separated and valorized. A reagent powder comprising sodium bicarbonate crystals made by such process.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a continuation of U.S. applicationSer. No. 12/991,350 which is a U.S. national stage application under 35U.S.C. §371 of International Application No. PCT/EP2009/055722 filed May12, 2009, which is a continuation of U.S. patent application Ser. No.12/126,651, filed May 23, 2008 and which claims priority benefit toEuropean Patent Application No. 08156095.5 filed on May 13, 2008, thewhole content of these applications being incorporated herein byreference for all purposes.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a method for the joint production of sodiumcarbonate and sodium bicarbonate out of trona ore.

BACKGROUND OF THE INVENTION

Trona ore is a mineral that contains about 90-95% sodium sesquicarbonate(Na₂CO₃.NaHCO₃.2H₂O). A vast deposit of mineral trona is found insouthwestern Wyoming near Green River. This deposit includes beds oftrona and mixed trona and halite (rock salt or NaCl) which coversapproximately 2,600 km². The major trona beds range in size from lessthan 428 km² to at least 1,870 km². By conservative estimates, thesemajor trona beds contain about 75 billion metric tons of ore. Thedifferent beds overlap each other and are separated by layers of shale.The quality of the trona varies depending on its particular location inthe stratum.

A typical analysis of the trona ore mined in Green River is as follows:

TABLE 1 Constituent Weight Percent Na₂CO₃ 43.6 NaHCO₃ 34.5 H₂O(crystalline and free moisture) 15.4 NaCl 0.01 Na₂SO₄ 0.01 Fe₂O₃ 0.14Insolubles 6.3

The sodium sesquicarbonate found in trona ore is a complex salt that issoluble in water and dissolves to yield approximately 5 parts by weightsodium carbonate (Na₂CO₃) and 4 parts sodium bicarbonate (NaHCO₃), asshown in the above analysis. The trona ore is processed to remove theinsoluble material, the organic matter and other impurities to recoverthe valuable alkali contained in the trona.

The most valuable alkali produced from trona is sodium carbonate. Sodiumcarbonate is one of the largest volume alkali commodities made in theUnited States. In 1992, trona-based sodium carbonate from Wyomingcomprised about 90% of the total U.S. soda ash production. Sodiumcarbonate finds major use in the glass-making industry and for theproduction of baking soda, detergents and paper products.

A common method to produce sodium carbonate from trona ore is known asthe “monohydrate process”. In that process, crushed trona ore iscalcined (i.e., heated) into crude sodium carbonate which is thendissolved in water. The resulting water solution is purified and fed toa crystallizer where pure sodium carbonate monohydrate crystals arecrystallized. The monohydrate crystals are separated from the motherliquor and then dried into anhydrous sodium carbonate. However, thesoluble impurities contained in the trona ore, tend to accumulate intothe crystallizer. To avoid build up of impurities, the mother liquormust be purged. The purge liquor, which represents important quantitiesfor industrial monohydrate plants, is commonly sent to evaporativeponds. The significant quantity of alkali which is contained in thepurge liquor is consequently lost. Moreover, the stocking of largequantities of purge liquors in evaporative ponds raise environmentalproblems, because of the scarce availability of new areas for stocking.

On the other side, sodium bicarbonate is a product with a wide range ofinteresting properties and a very wide range of applications from hightech ingredients for the pharma industry to the human food and animalfeed, and to the use in flue gas treatment. In flue gas treatment sodiumbicarbonate is most likely among the most efficient chemicals for theremoval of a wide range of pollutants (most notably the acidic one), andits use is limited only by the competition of less efficient but muchcheaper chemicals such as lime or even limestone.

The production of sodium bicarbonate is currently almost entirely madeby the carbonation of sodium carbonate. In Europe, the carbonation isusually performed in situ in the soda ash plants from CO₂ coproducedduring the production of soda ash (mainly the CO₂ generation in the limekilns). In USA, the carbonation is usually made in separate plants whichpurchase independently the soda ash and the CO₂ and combine them.

Because of the nature of this most important process for the bicarbonateproduction, the price for bicarbonate is above the price of the sodaash. With such economics the uses of bicarbonate will always be limitedby the competition of cheaper substitutes, most notably in the flue gastreatment.

US2003/0017099 discloses a process for the joint production of sodiumcarbonate and bicarbonate, according to which solid trona is dissolvedin water and the resulting water solution is fed into a monohydratecrystallizer in order to produce sodium carbonate. The purge liquor isintroduced into a decahydrate crystallizer and the decahydrate crystalsconverted into sodium bicarbonate. It has been observed that thisprocess is not efficient when the purge liquor, depending on the tronasource, contains high levels of impurities. In particular, the sodiumchloride content of the trona ore can vary depending on the precisetrona vein which is exploited. High levels of sodium chloride in thepurge liquor prevent smooth crystallization of decahydrate.

SUMMARY OF THE INVENTION

The invention aims on one side at reducing the amount of alkali lost inthe evaporative ponds and on the other side at producing bicarbonatefrom trona in a smooth and inexpensive way, thereby opening newapplications for the sodium bicarbonate.

Accordingly, the invention concerns a process for the joint productionof sodium carbonate and sodium bicarbonate, wherein:

-   -   a solid powder derived from sodium sesquicarbonate, having a        mean particle diameter comprised between 0.1 and 10 mm is        dissolved in water;    -   the resulting water solution is introduced into a crystallizer,        wherein a first water suspension comprising sodium carbonate        crystals is produced;    -   the first water suspension is subjected to a separation, in        order to obtain crystals comprising sodium carbonate on the one        hand, which are valorized, and a mother liquor on the other        hand; and    -   a part of the mother liquor is taken out of the crystallizer and        put into contact with a gas comprising carbon dioxide, in order        to produce a second water suspension comprising sodium        bicarbonate crystals, which are separated and valorized.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of the preferred embodiments of theinvention, reference will now be made to the accompanying drawing inwhich:

FIG. 1 illustrates a process flow diagram of a process according to anembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process according to the invention allows the joint production ofsodium carbonate and sodium bicarbonate, out of sodium sesquicarbonate.Sodium sesquicarbonate containing intrinsically both sodium carbonateand bicarbonate, this process valorizes in an optimum way the rawmaterials which are consumed.

In the process according to the invention, solid powder derived fromsodium sesquicarbonate is dissolved in water. The expression “derivedfrom sesquicarbonate” means that the powder can consist essentially ofsesquicarbonate, but can also consist of a product which results from adirect transformation of sesquicarbonate. For example, thetransformation can be a calcination which transforms the sesquicarbonateessentially in sodium carbonate. The sesquicarbonate can have differentorigins. It can be produced artificially out of different sodiumsources. However, it is recommended that sesquicarbonate comes from anatural trona ore. In this recommended embodiment purification of thewater solution obtained after the dissolution of the solid powder inwater will in general be necessary, in order to purify it from the mainimpurities contained in the ore. The purification generally involvessettling and filtration steps, to allow insolubles to separate from thewater solution. It also involves generally the use of reagents in orderto remove organic matters still contained in the purified watersolution. Active carbon is an example of such reagent.

The water in which the solid powder derived from sesquicarbonate isdissolved can be fresh water. However water has to be understood in awide sense. The water can contain recycled water solutions alreadycontaining alkalis, coming from the process according to the inventionor from other processes. The water can also comprise mother liquors(crystallization waters) produced downstream of the process according tothe invention, when sodium carbonate and bicarbonate are crystallized,for instance. The process is also suited when the water is a mine water.By mine water is understood the water solution which is formed whenwater is directly injected into the trona ore deposits, whereby, oncontact with the water, some ore dissolves in it.

The mean particle diameter of the powder which is dissolved in the wateris comprised between 0.1 and 10 mm. Powders having a mean diameter below0.1 mm frequently contain too much impurities, for instance when thesesquicarbonate is a trona ore, whereas powders having a mean diameterabove 10 mm tend to be difficult to handle and dissolve in water. Themean diameter is the D₅₀ which is the diameter such that half of theparticles, in weight, have a diameter lower than the specified value.For non spherical particles, the diameter is the equivalent one, that issix times the value of the volume of the particles divided by itsexternal area.

The powder which derives from sesquicarbonate can consist essentially ofsesquicarbonate and the impurities accompanying it, as in the embodimentwherein the source of sesquicarbonate is natural trona ore.

In a recommended embodiment of the invention, the powder derived fromsesquicarbonate is calcined sesquicarbonate. In this embodiment, thesesquicarbonate is first calcined, preferably at a temperature comprisedbetween 100 and 400° C., before its dissolution in water. Duringcalcination, the sodium sesquicarbonate in the trona ore breaks downinto sodium carbonate, carbon dioxide and water. Also, calcinationreleases some of the organics associated with trona or trona shale.

The quantity of powder derived from sesquicarbonate which is dissolvedinto water is regulated in order to obtain a resulting water solutioncontaining enough sodium carbonate and bicarbonate to allow smoothcrystallization of both chemicals in the later stages of the process. Itis recommended that the resulting water solution contains at least 15%,preferably 20%, most preferably 25% in weight of sodium carbonate.

The crystallizer into which the resulting water solution is introducedmust be able to crystallize sodium carbonate. The crystallized sodiumcarbonate can be in different hydration forms: monohydrate, decahydrate,. . . or can be anhydrous.

In a preferred embodiment of the invention, the sodium carbonatecrystals produced in the crystallizer are in the monohydrate form. Thecrystallizer is then part of what is commonly referred to as the“monohydrate process”. In the monohydrate process, crushed trona ore iscalcined at a temperature between 125° C. and 250° C. to convert sodiumbicarbonate into sodium carbonate and form crude soda ash. The resultingcrude sodium carbonate and the remaining organics are then dissolved inwater. After dissolving the calcined trona, any undissolved solids areremoved and the solution is treated with activated carbon to remove someof the organics present in the solution. The solution is then filtered.One of the advantages of the monohydrate process is that calcined tronadissolves faster than raw trona. Another advantage is that calcinedtrona can produce more concentrated sodium carbonate solutions, whoseconcentrations can reach about 30%, while dissolved raw trona resultsinto solutions having only about 16% sodium carbonate plus 10% sodiumbicarbonate. The filtered solution of sodium carbonate is fed to anevaporative crystallizer where some of the water is evaporated and someof the sodium carbonate forms into sodium carbonate monohydrate crystals(Na₂CO₃.H₂O). A slurry containing these monohydrate crystals and amother liquor is removed from the evaporators, and the crystals areseparated from the mother liquor. The crystals are then calcined, ordried, to convert it to dense soda ash. The mother liquor is recycledback to the evaporator circuit for further processing into sodiumcarbonate monohydrate crystals.

In the process according to the invention, the composition of the motherliquor which is put into contact with carbon dioxide can vary accordingto the crystallization conditions. In general, it is recommended thatthe mother liquor contains a sufficient quantity of sodium carbonate.

In a recommended embodiment of the process according to the invention,the mother liquor contains at least 175 g/kg, preferably 190 g/kg, morepreferably 205 g/kg, most preferably 220 g/kg of sodium carbonate. It ishowever recommended that the mother liquor does not contain more than250 g/kg, preferably not more than 240 g/kg of sodium carbonate. It isalso recommended that the mother liquor does not contain more than 30g/kg, preferably 20 g/kg, more preferably 15 g/kg, most preferably 10g/kg of sodium bicarbonate. It is additionally recommended that themother liquor contains from 3 to 6, preferably from 4 to 5 equivalent/kgtotal alkali content. This means that one kg of mother solution containsadvantageously from 3 to 6, preferably from 4 to 5 moles on ion Na⁺,whether coming from sodium carbonate or sodium bicarbonate.

The process according to the invention allows to directly produce fairlypure sodium bicarbonate crystals out of quite impure mother liquors. Themother liquor is even advantageously a purge stream from thecrystallizer, used to maintain the concentration of impurities in thecrystallizer below a threshold value.

In an advantageous embodiment of the invention, the mother liquorcontains at least 10 g/kg, preferably 20 g/kg, most preferably 30 g/kgof NaCl.

In another advantageous embodiment of the invention, the mother liquorcontains at least 1 g/kg, preferably 4 g/kg, most preferably 8 g/kg ofNa₂SO₄.

In still another advantageous embodiment of the invention, the motherliquor contains at least 0.5 g/kg, preferably 0.6 g/kg of Si (counted assilica).

It is however recommended in those advantageous embodiments that themother liquor does not contain more than 60 g/kg, preferably not morethan 50 g/kg of sodium chloride. It is also recommended that the motherliquor does not contain more than 20 g/kg, more preferably 15 g/kg ofsodium sulfate and not more than 1.5, preferably 1 g/kg of silica.

In those advantageous embodiments, it has been observed that theproduced sodium bicarbonate crystals contain much less impurities thanthe mother liquor. It is advantageous that the crystals contain lessthan 0.1 g/kg Na₂SO₄, less than 1 g/kg NaCl and less than 5 g/kg silica.

In the process according to the invention, the gas comprising carbondioxide must react efficiently with the mother liquor in the gas liquidreactor. To that end, it is recommended that the gas comprises at least20% in weight, advantageously 40%, preferably 60%, more preferably 80%CO₂. It is particularly efficient to use pure (100%) CO₂. It is alsorecommended to use a well stirred gas liquid reactor, comprising a gasinjector able to distribute the gas homogeneously into the reactor. Theliquid constitutes advantageously the continuous phase inside thereactor, the gas being injected at the bottom and moving upwards. Thereactor comprises preferably cooling means, to counteract theexothermicity of the reaction. The CO₂ can have different origins. Inone recommended embodiment, the CO₂ comes from a natural gas plant,after having been concentrated for example through an amine process.Preferably, the CO₂ comes from the monohydrate soda ash plant, forinstance from the calciners used to calcine the trona.

The temperature inside the gas liquid reactor is preferably between 60and 80° C., more preferably between 65 and 75° C. The temperature of themother liquor when it is introduced into the reactor is advantageously alittle higher, preferably between 80 and 95° C.

In order to obtain a water suspension comprising enough sodiumbicarbonate crystals, it is preferable to maintain a residence time inthe gas liquid reactor greater than 10 minutes, more preferably greaterthan 20 minutes.

The (second) water suspension produced into the gas liquid reactor issubjected to a separation. The separation of the crystals from thesuspension can be carried out by any appropriate mechanical separatingmeans, for example by settling, by centrifugation, by filtration or by acombination of these three separating means. The sodium bicarbonatecrystals are finally dried and packed.

The process according to the invention is particularly effective toproduce crystals with a median diameter (D₅₀) between 75 and 250 μm,preferably between 80 and 150 μm. D₁₀ diameters are preferably between40 and 100 μm, whereas D₉₀ diameters are preferably between 175 and 500μm. D_(x) is the diameter value such that×percent of the particles havea diameter lower than the value. When the particles have anapproximately spherical shape, the diameter is the diameter of theparticle. For irregular shapes, the diameter is six times the volume ofthe particle divided by its outer surface.

The sodium bicarbonate crystals produced by the process according to theinvention have a very special structure: they contain impurities at aparticular, however low, level. This level is higher than that ofconventional sodium bicarbonate crystals for instance produced out ofcommercial sodium carbonate. Those impurities are a kind of memory inthe bicarbonate crystals of the composition of the mother liquor. Theusefulness of those impurities is not yet fully experienced, but theirconcentration corresponds to the level of many additives. Positiveimpact on storage and flowability of powders of such crystals can beexpected. The crystals have also a unique granulometry. Moreover, theyare extremely advantageous for many applications, in which cost is amajor aspect.

The invention concerns also a reagent powder comprising sodiumbicarbonate crystals obtainable by the process according to theinvention. The crystals of such reagent powder comprise preferably from0.1 to 1 g/kg NaCl, and/or from 0.01 to 0.1 g/kg Na₂SO₄ and/or from 0.5to 5 g/kg silica.

Such reagent powders are particularly suited for the removal ofpollutants from gases.

Consequently, the invention concerns also a process for treating a gascontaining noxious pollutants, preferably HCl and/or SO₂ according towhich a reagent powder according to the invention is injected in thegas, the pollutants react with the reagent and the product of thereaction is separated from the gas. The separation of the products ofthe reaction can most simply be performed by filtration, using bagfilters or electrostatic filters. In this process, it is recommendedthat the temperature of the gas is above 100° C., preferably 110° C.,more preferably 120° C., most preferably 130° C. At those temperatures,the sodium bicarbonate quickly decomposes into sodium carbonate having ahigh specific surface and thus high reactivity. The decomposition occurswithin seconds, in the gas treatment duct. The reagent is injected inthe dry or semidry state. By semidry state injection is understoodinjection of fine droplets of a water solution or preferably suspensionof the reagent into a hot gas, having a temperature above 100° C. Thesolution or suspension evaporates immediately after its contact with thehot gas.

In the process for the joint production of sodium carbonate and sodiumbicarbonate crystals according to the invention, the gas comprising CO₂is preferably produced by indirect calcination of a compositionreleasing CO₂ upon calcination, preferably a composition comprising analkali bicarbonate, more preferably sesquicarbonate or trona.Calcination of trona is advantageously operated between 140 and 180° C.By indirect calcination is meant calcination wherein the composition tobe calcined is not in direct contact with the heat source utilized towarm the calciner. This is indeed the situation in conventionalcalciners, wherein the composition is in direct contact with thecombustion gases produced by the burning fuel. In this embodiment, it isrecommended to use steam heated calciners, wherein the steam iscirculated into pipes, and the composition, preferably trona, is heatedby contact with the exterior surface of the pipes. The steam isadvantageously produced by electricity and steam cogeneration. It hasbeen observed that the gas comprising CO₂ which is produced that way,after suitable drying for instance by a condensing step, has an elevatedconcentration in CO₂, typically more than 80% in volume, preferably morethan 90%, most preferably more than 95%. The CO₂ has also a greatpurity. Thanks to those properties, a gas comprising CO₂ produced thatway is especially suitable for the production of sodium bicarbonate outof a water solution comprising sodium carbonate.

Consequently, the invention concerns finally also a process for theproduction of sodium bicarbonate crystals, according to which:

-   -   a composition releasing CO₂ upon calcination is indirectly        calcined in order to produce a gas comprising CO₂;    -   a water solution comprising sodium carbonate is put into        contact, in a gas liquid contactor, with the gas comprising CO₂,        in order to produce a water suspension comprising sodium        bicarbonate crystals, which are separated.

In this process, the solution comprising sodium carbonate comprisespreferably at least 175 g/kg of sodium carbonate, and the gas comprisingCO₂ comprises at least 90% CO₂. The sodium carbonate is preferablyproduced by the monohydrate process described in this specification.Other preferred embodiments of the process for the joint production ofsodium carbonate and sodium bicarbonate crystals described above arealso advantageously adapted to this process for the production of sodiumbicarbonate.

The annexed figure (FIG. 1) illustrates a particular embodiment of theinvention. Crushed sodium carbonate crystals 1, originating fromcalcined trona ore, and water 2 are introduced in a leaching tank 3. Theresulting water solution, containing insolubles in suspension, isfiltered and purified in a purification unit 5. The purified watersolution 6 is introduced into a monohydrate crystallizer 7, wherein asuspension 8 containing sodium carbonate monohydrate crystals isproduced. Those crystals 10 are separated from the suspension in aseparator 9. The resulting mother liquor 11 is sent back to thecrystallizer 7. A purge stream 12 from the crystallizer 7 is carbonatedin a reactor 13, fed by carbon dioxide 14. A water suspension 15comprising sodium bicarbonate crystals is extracted from the reactor 13.The crystals 22 are finally separated in a filter 16. The second motherliquor 17 is debicarbonated with vapor 20 and then sent to a storagepond. Carbon dioxide 19 is advantageously recycled.

EXAMPLES

Details and particularities of the invention will appear from thedescription of the following examples.

Example 1

Crushed trona ore originating from Wyoming was used as feed material ina monohydrate process for the production of sodium carbonate.Accordingly, the crushed trona ore was calcined at a temperature of 170°C. The resulting sodium carbonate was leached in a quantity regulated inorder to get a water solution containing 30% (weight) of sodiumcarbonate. The resulting water solution was then filtered, purified andintroduced into a crystallizer, according to the monohydrate process. Afirst water suspension comprising sodium carbonate monohydrate crystalswas produced in the crystallizer. The suspension was submitted to aseparation, resulting in sodium carbonate monohydrate crystals (whichare further processed into dense anhydrous sodium carbonate crystals) onone side and a (first) mother liquor on the other side. Part of themother liquor was then taken out of the crystallizer, as part of a purgestream. The composition of the mother liquor is given in TABLE 2. Themother liquor was stored in a tank and heated at 87° C. This motherliquor was introduced from the tank into a lab-scale, atmosphericpressure gas-liquid reactor, at a flow rate of 1.6 kg/h. The reactor wasagitated and maintained at 70° C. A carbon dioxide gas stream (100%CO₂), saturated at about 40° C. was introduced into the reactor at aflow rate of 0.8 m³/h and approximately atmospheric pressure. Residencetime into the reactor was calculated as approximately 1_hour. A secondwater suspension comprising sodium bicarbonate crystals was produced andextracted from the bottom of the reactor. The crystals were separatedfrom the suspension. The resulting second mother liquor had thecomposition given in TABLE 3. Size and composition of those crystals aregiven in TABLE 4.

Example 2

In Example 2, it was processed as in Example 1, except that anotherspecimen of mother liquor was submitted to carbonation, with slightlyhigher alkali content. The residence time was also increased to 2 hours.The results are given in TABLES 2 to 4. The residual sodium carbonatecontent of the second mother liquor was much higher than in Example 1.The sodium bicarbonate crystals comprised also more fine particles(greater span).

Example 3

In example 3, it was operated as in Example 2 (2-hour residence time)and the same specimen of mother liquor was used than in Example 2, butwas further slightly diluted with water, in order to bring its alkalicontent back to the value of Example 1. The results are given in TABLES2 to 4. The residual sodium carbonate content of the second motherliquor was back to the value of Example 1.

Example 4

In Example 4, it is operated as in Example 1, except that the purgestream from the crystallizer is sent into a pilot scale reactor, at aflow rate of 320 kg/h. Carbon dioxide having a concentration of 75% isintroduced into the reactor at a flow rate of 18.5 Nm³/h, and at apressure of 2.5 absolute bars. The crystals separated from the extractedsuspension have approximately the same composition as those ofExample 1. Their diameters have a D₁₀ of 60 μm, a D₅₀ of 120 μm and aD₉₀ of 200 μm.

TABLE 2 Example 1 Example 2 Example 3 NaHCO₃ 9 g/kg 14 g/kg  9 g/kgNa₂CO₃ 229 g/kg 239 g/kg  229 g/kg NaCl 35 g/kg 39 g/kg  36 g/kg Na₂SO₄9 g/kg 10 g/kg  9 g/kg Ca 8 mg/kg Mg 0.7 mg/kg Fe 0.1 mg/kg Al 0.3 mg/kgSi 800 mg/kg Total Organic Carbon 613 mg/kg H₂0 718 g/kg 698 g/kg  717g/kg

TABLE 3 Example 1 Example 2 Example 3 NaHCO₃ 93 g/kg 63 g/kg 96 g/kgNa₂CO₃ 46 g/kg 115 g/kg  44 g/kg NaCl 39 g/kg 45 g/kg 32 g/kg Na₂SO₄ 10g/kg 12 g/kg  7 g/kg Ca 0.5 mg/kg Mg 0.2 mg/kg Fe <0.04 mg/kg Al <0.04mg/kg Si 400 mg/kg Total Organic Carbon 602 mg/kg H2O 812 g/kg 765 g/kg 821 g/kg 

TABLE 4 Example 1 Example 2 Example 3 NaHCO₃ 977 g/kg 989 g/kg 985 g/kgNa₂CO₃ 18 g/kg 9 g/kg 10 g/kg NaCl 0.3 g/kg Na₂SO₄ 80 mg/kg Ca 23 mg/kg43 mg/kg 37 mg/kg Mg 2.5 mg/kg Fe 0.4 mg/kg Al 1.0 mg/kg Si 2.6 g/kg D₁₀22 μm 21 μm 40 μm D₅₀ 89 μm 80 μm 110 μm D₉₀ 222 μm 294 μm 220 μm Span2.2 3.4 1.6

1-20. (canceled)
 21. A method for reducing the amount of alkali lost inevaporative ponds which are fed with a purge liquor containing suchalkali, comprising: contacting said purge liquor with a gas comprisingcarbon dioxide.
 22. The method according to claim 21, wherein the purgeliquor is a part of a mother liquor comprising sodium carbonate takenout of a sodium carbonate crystallizer.
 23. The method according toclaim 22, wherein the sodium carbonate content in the mother liquor isat least 175 g/kg.
 24. The method according to claim 22, wherein thesodium carbonate content in the mother liquor is not more than 250 g/kg.25. The method according to claim 22, wherein the mother liquor does notcontain more than 30 g/kg of sodium bicarbonate.
 26. The methodaccording to claim 21, which produces a water suspension comprisingsodium bicarbonate crystals.
 27. The method according to claim 26,wherein sodium bicarbonate crystals are separated from the watersuspension to form a second mother liquor.
 28. The method according toclaim 27, wherein the second mother liquor is debicarbonated with vaporand then sent to a pond.
 29. A method for treating a purge streamcontaining an inorganic salt, comprising: treating said purge streamwith gaseous carbon dioxide.
 30. The method of claim 29, wherein saidpurge stream comprises sodium carbonate.
 31. A method for extending thelife of tailings ponds produced from purge streams containing inorganicsalts, which method comprises treating such purge stream with gaseouscarbon dioxide.
 32. The method of claim 31, wherein the purge stream istreated prior to it being deposited in the tailings pond.
 33. The methodof claim 31, wherein such purge stream is a soda ash purge stream. 34.The method of claim 31, wherein such inorganic salt comprises sodiumcarbonate.